The present application relates to current sensors and, more particularly, to a novel current sensor utilizing a piezoelectric element and having an output proportional to the first power of a current to be measured.
It is well known to sense the magnitude of a flowing current by means of a current shunt or a current transformer, both of which components are relatively expensive, relatively bulky and which are not integratable with any associated electronic circuitry. An integratable current sensor 10 has been described and claimed in copending application Ser. No. 432,207, filed Oct. 1, 1982, by Henry A.F. Rocha, assigned to the assignee of the present application and incorporated herein by reference in its entirety. In this sensor (see FIG. 1), a piezoelectric member 11 is provided with electrodes 12a and 12b on the opposed surfaces thereof. Electrodes 12a and 12b are respectively coupled to lead conductors 14a and 14b for providing a sensor output voltage. The electrodes 12a and 12b are respectively in abutment with first and second insulative layers 16a and 16b, serving to insulatively position the electrode-supporting piezoelectric member 11 between a pair of conductors 16a and 16b carrying an identical current I in the same direction relative to the piezoelectric element. The conductors 18a and 18b may be supported by insulative means 20 and the entire conductor-insulator-piezoelectric element assembly may be fabricated upon a supportive substrate 22. Sensor 10 provides an output voltage, between leads 14a and 14b, responsive to the magnitude of the compressive force acting upon piezoelectric element 11, which force is proportional to the product of the currents flowing in the same direction in conductors 18a and 18b. By connecting conductors 18a and 18b such that the same current flows through both conductors, the sensor is proportional to the square of the current I. While it may be desirable, in certain instances, to have a sensor output proportional to the square of the sensed current, it is highly desirable to provide a sensor which is responsive to the first power of the current, i.e. a sensor having an output voltage V=kI, where k is a proportionality constant. Further, because the piezoelectric element 20 supports some of the uniform force between conductors 18 and insulators 16 support the balance of the forces, only a fraction of the total force is supported by the piezoelectric element 20, and that fraction depends upon the relative compliance of the piezoelectric element, the conductors and the insulators. Thus, the output is a function of the sensor geometry, which is affected by the temperature to which the sensor is subjected. It is further desirable to provide a sensor having an output variability less subject to temperature and geometry considerations.